In the petroleum industry it is often advantageous to drill boreholes along non-vertical, directional paths in order to optimally produce hydrocarbons. These directional paths, composed of a set of "curved" segments and "straight" segments, are most often drilled with a drilling assembly where the drill bit is powered by a mud motor. The drilling assembly is composed of a drill bit and mud motor with one or more "bends" immediately above, below, or intermediate the motor. When the bit is being steered in a desired direction, the entire drill string is not rotated in order to maintain the "bends" and the motor directed in the proper orientation.
This type of system has several inherent disadvantages. The mud motor is expensive to manufacture and maintain. It is a failure prone piece of equipment. The non-rotating drill string also causes cuttings to accumulate on the low side of the borehole which may inhibit the removal of the drill string. Non-rotation of the drill string results in high frictional contact between the wellbore wall and the drill string which inhibits the smooth application of an axial force to the drill bit which is needed in order to drill efficiently. In addition, the drill string tends to "stick" in the borehole and does not slide down freely. Moreover, as soon as sufficient force is added to the drill string to cause it to break the static friction and slide, it often slides too rapidly and overpowers the motor. This may cause the motor to stall or to apply too much torque to the drill string so that the desired orientation is lost. Either consequence results in wasted time and money by requiring that the drilling process be restarted. For these reasons it is desirable to continuously rotate the drill string while directing the bit along the desired directional path.
In anticipation of these difficulties and in order to minimize them, many times an "unaggressive" drill bit is used which results in the penetration rate being slower than it would be if the optimum drill bit were used. Again, this results in an inefficient drilling operation.
Because of their high initial manufacturing cost, high maintenance cost, low reliability, and overall drilling inefficiencies when using mud motor systems, it is anticipated that mud motor drilling assemblies for directional work will become obsolete.
There are a number of new rotary steerable systems currently under development. Many of the systems that are receiving the most attention are very sophisticated systems that are also likely to have high initial manufacturing costs and possibly be less reliable than a simple system.
There are also a number of less complicated devices that have been proposed to allow the drill bit to be directed along a curved path while rotating the drill string. Some of these tools, such as that disclosed in U.S. Pat. No. 5,213,168 which is assigned to Amoco Corporation, require that the drill string be withdrawn from the borehole when it is desired to change from drilling along a curved path to drilling along a straight path. This is a relatively expensive and inefficient process. Other devices, such as that illustrated in U.S. Pat. No. 5,265,682 to Russell et al., allow the drilling mode to be shifted from curved to straight without withdrawing the apparatus from the wellbore. These tools are also relatively complicated, expensive, and failure prone. While there are some relatively simple designs, such as that shown in U.S. Pat. No. 4,895,214 to Schoeffler, for shifting the drilling mode from curved to straight, those do not appear to have been reduced to practice or been commercially successful for one reason or another. Thus, there is a continuing need for improvement.